1. Field of Invention
The present invention relates to cleaning vacuum chambers and vacuum analytical instruments such as Scanning Electron Microscopes (SEM), Scanning Electron Microprobes, Transmission Electron Microscopes (TEM) and other charge particle beam instruments that are subject to contamination problems from hydrocarbons.
2. Description of Prior Art
Electron microscopy is used to detect, measure, and analyze constituents present in very small areas of materials. Hydrocarbon contaminants adsorbed on the surface or surface films interacting with the incident electron probe beam can distort the results. The distortion may take the form of deposits of polymer in the scanned area, a darkening of the scanned area, a loss of resolution, or other artifacts. Deposits created by the interaction of the probe beam with the surface specimen also may interfere with the probe beam or emitted electrons and x-rays and thus adversely affect accurate analysis. Deposits also add uncertainty to SEM measured line widths for semiconductor device critical dimension metrology. These Hydrocarbons are present in trace levels in ordinary room air and come from living organisms and man-made material. All surfaces exposed to room air at atmospheric pressure accumulate these hydrocarbons. In the semiconductor industry said contamination is known as Atmospheric Molecular Contamination or “AMC”. Reducing and controlling AMC is an active area of concern for semiconductor manufacturers as device dimensions get smaller. Surfaces are thither hydrocarbon contaminated by touching, the use of high vapor pressure materials in vacuum system, or in general “poor vacuum practices”.
Another problem is the condensation of pump oils on the windows of the x-ray and electron detectors distorting results. The most serious problem of this type is the absorption of low-energy x-rays from Be, C, N, O and F by oil films which can prevent measurement of these elements by X-ray emission spectroscopy.
Contaminants typically are introduced by one of four ways including on the specimen, on the specimen stage, carried into the chamber by the evacuation system, or are present on the internal components of the instrument. Contaminants introduced from the evacuation system can be reduced by trapping, by purging, or by using cleaner pumps. Once present inside the chamber, these contaminants reside on the chamber surfaces and can be removed only slowly and with low efficiency by the high vacuum pump.
Inorganic specimens (metals, ceramics, semiconductors, etc.) may carry contaminants into the chamber. These may be part of the specimen, residues from sample preparation techniques or be caused by improper sample handling or storage techniques. In addition, clean surfaces will accumulate a surface film of hydrocarbon scum if left exposed to ordinary room air for any length of time. The sources of these hydrocarbons are most any living thing, organic object, or other source of hydrocarbon vapors in the vicinity. While the part of the films created in these processes dissipate under vacuum conditions, a small amount generally remains on surfaces and is sufficient to cause problems when the specimen is subsequently examined in the analytical instruments listed.
These residues are widely distributed and generally are at low concentrations on the various surfaces. Some of the contaminant molecules become mobile in the vacuum environment. At high vacuum the mean free path of molecules once vaporized is comparable to or longer than the dimensions of the vacuum chamber of these instruments. The contaminants move in the vapor phase from surface to surface in the vacuum environment and are attracted to any focused electron probe beam, forming deposits through an ionization and deposition process. These deposits are also degraded into black carbon deposits by vacuum ultraviolet VUV light and intense laser light sources. Since these contaminants can travel large distances within the vacuum chamber and over the surface of a specimen, it is important to remove or immobilize these species as much as possible prior to an analysis without disturbing the microstructure of the specimen.
Several patents have previously described methods of reducing contamination in electron microscopes. Hahn et al in U.S. Pat. No. 3,148,465 (1968) described a method of immobilizing the Hydrocarbon by exposing it to radiation near the specimen to produce an adsorbing effect on the surrounding surfaces. A device for cleaning electron microscope stages and specimens is described in U.S. Pat. No. 5,510,624 (Zaluzec, 1995) for analytical electron microscopes. That apparatus uses a plasma generating chamber and an airlock to allow the specimen and stageS to be placed into the plasma chamber for cleaning. It may be connected with the analytical chamber of the analytical electron microscope. Glow-discharge and plasma cleaning devices and cleaning methods for electron optics are described in U.S. Pat. Nos. 5,312,519 (Sakai et al.), 5,539,211 (Ohtoshi et al.) and 4,665,315 (Bacchetti et al.). These three patents use either direct or remote plasma cleaning to clean the electron optics of the instruments.
Vane disclosed in U.S. Pat. Nos. 6,105,589, 6,452,315, and 6,610,257 the technology used by XEI Scientific, Inc. in the Evactron® De-Contaminator systems that have been sold for cleaning electron microscopes and other vacuum systems since 1999. These patents describe an oxidative cleaning system and apparatus using low powered RF plasma to produce oxygen radicals, an active neutral species, from air to oxidize and remove these hydrocarbons. This plasma excited system works well, but suffers from a Nitrogen ion problem as disclosed in the first patent (U.S. Pat. No. 6,105,589) and solved by using a very low energy plasma and special electrode (U.S. Pat. No. 6,610,257) for dissociation of the oxygen in air. The reaction with oxygen radicals to produce CO, CO2, H2O and other volatile oxides such as short chain alcohols and ketones are the most important for the cleaning and removal of hydrocarbons by the vacuum pump. These reaction products are quickly removed as gases from the vacuum system. The ions and electrons produced by the plasma are not needed as the reactive species for hydrocarbon removal. A disadvantage of plasma is that they produce UV light, ions, and electrons that polymerize the hydrocarbons and make them harder to remove. In the absence of nitrogen ions or other reactive species that destroy O radicals, O radicals are long lived and have the ability to do isotropic cleaning on all surfaces in the chamber. Another disadvantage of the Evactron device is that it will not produce O radicals at pressures below 10-4 Torr which keeps the Evactron from cleaning when the instrument is at high vacuum. Another disadvantage is that the plasma produces high levels of free electrons in SEM imaging while the Evactron plasma is operating. (® XEI Scientific, Inc.)
In the operation of the Evactron® systems it was noticed that the UV light from plasma source had a positive effect on the cleaning efficiency of system, thus further investigation was done. It has been well documented that UV light can be used to produce Ozone and then disassociate Ozone to make O radicals for removing semiconductor photo resist and other accumulated reaction products in semiconductor production. (Rhieu U.S. Pat. No. 6,143,477), (Parke U.S. Pat. No. 6,098,637). The usual method is to produce Ozone either by disassociation of Oxygen by electrical discharge, in a plasma, or by UV excitation with wavelengths below 193 nm. The O radicals (O1 atoms) then react with O2 to form O3 Ozone. The production of Ozone is an exothermic chemical reaction and energy is released. It is well known in chemical physics theory that this reaction requires a third body, another molecule or atom, to carry away this extra energy as kinetic energy, or the newly formed ozone molecule will promptly disassociate. As practical matter this means that Ozone is not formed in significant quantities at pressures below about 133 Pa (Pascal) or 1 Torr. Thus to form Oxygen radicals for use in a vacuum system all that is required are pressures below 1 Torr, O2, and source of energy for disassociation. The Evactron De-Contaminator uses an RF plasma to produce Oxygen radicals. But UV light can also be used to make Oxygen radicals. UV light from 193 nm to 150 nm is strongly absorbed by Oxygen O2 to produce O radicals. UV light between 220 nm and 240 nm weakly photo disassociates Oxygen.
The use of UV light to excite Oxygen for cleaning and ashing has been done by others. Spill (U.S. Pat. No. 7,005,638) discloses directing an electron beam and UV light beam simultaneous on the specimen surface to reduce contamination. Van Schaik et al (U.S. Pat. No. 6,724,460) uses the interaction of the EUV beam with low concentration of oxygen to produce oxygen radical for cleaning a lithographic projection apparatus. Agarwal (U.S. Pat. No. 6,649,545) discloses using UV lamps to keep active species produced in a upstream plasma active for plasma processing. Rheiu (U.S. Pat. No. 6,143,477) discloses the use of two UV lamps to make Oxygen radicals for cleaning/ashing of semiconductor wafers. The first is used to make Ozone with UV<190 nm and the second (about 250 nm) to disassociate the Ozone to make radicals.
It is an object of the present invention to provide an improved method for cleaning the specimen chamber, specimen stage and a specimen in the vacuum system of an electron microscope or similar analytical instrument using an electron beam such as a scanning electron microprobe instrument or Focused ion beam instrument. It is another object of the present invention to produce more O radicals from air by completely avoiding the production of nitrogen ions. It is another object of the present invention to use single UV lamp and to avoid the production of ozone by using vacuum pressures too low to sustain ozone formation. It is another object of the present invention to produce oxidation and ashing without the need to expose the surfaces or chamber to high intensity UV light or plasma. It is another object of the present invention to provide a method for cleaning said instruments that can be operated at lower pressures than plasma methods thus alleviating the need to raise to pressure to plasma operation pressures, and allow instrument operation during cleaning. It is another object of the present invention to provide a method for cleaning said instruments that does not produce free electrons and ions in a plasma that would interfere with electron detection during instrument operation. It is another object of the present invention to provide a cleaning system that is small and that can be mounted on a standard chamber port of the electron microscope without mechanical interference from other devices and parts of the electron microscope. These improvements results in a cleaning system that is faster and cleans the specimen chamber, stage, and specimen of the analytical instrument better than previous arrangements. The result of a cleaner specimen, specimen chamber and stage is that the deposition of hydrocarbon polymer on the scanned area is reduced or eliminated resulting in better measurements. Another result of cleaner specimen chambers is that the condensation and adsorption of hydrocarbons on detector windows is reduced which allows the passage of more low energy x-rays and electrons through these windows.